1. Field of the Invention
The present invention relates to wireless networks, and more particularly, to the benefits of scheduling transmissions in such networks.
2. Related Art
Wireless local networks (WLANs) based on the IEEE 802.11 standard have proven to be popular. IEEE 802.11 is a wireless standard related to the IEEE 802.3 standard established for wired Ethernets. In contrast to wired networks, an IEEE 802.11 WLAN must conserve the limited bandwidth presented by a wireless transmission medium. Accordingly, a set of rules in the IEEE 802.11 standard is dedicated to medium access control (MAC), which governs accessing the wireless medium and sending data through it.
The 802.11 rigidity and power allocation limits present severe challenges to users during network deployment and modification. Even if careful network planning is implemented, there may still be loss of bandwidth due to unpredictable circumstances such as a subscriber's movement and activity level. Further, with existing WiFi chipsets, bandwidth may not be fully utilized due to various factors, including unpredictable communication traffic and “hidden node” situations (which will be described below).
For example, FIG. 1A illustrates an ideal situation where each cell 10 has a circular coverage area. However, in reality, the coverage area of each cell 10 is not a circle. For example, in an enterprise application, such as in a building with large numbers of walls and offices, numerous APs and STAs are needed to allow STAs to transfer information between each other. The walls and other barriers result in non-uniform coverage areas for each cell 10.
FIG. 1B shows coverage areas or cells 20 in a practical WLAN environment. As seen, the coverage areas are no longer uniform circles, but are irregular having areas of broader coverage (the peaks) and areas of lower coverage (the nulls). For example, long peaks 25 may correspond to long hallways in the building. Because cells 20 do not have uniform coverage, “holes” 30 exist in the network, where communication is not possible. Holes 30 do not necessarily represent areas where no frames can be sent and received; however, only a small percentage of dropped frames may be enough severely disrupt TCP/IP behavior, thereby effectively ending communication ability within that area.
A possible solution to “fill” holes 30 may be to increase the density of the APs in the WLAN, i.e., move the APs closer to each other, which requires more APs for the same outer coverage area. However, increasing the density of the APs will result in increased interference between APs and STAs, while also increasing the cost of the system. Consequently, in order to reduce interference, the transmit power of the APs must be reduced. But, this may again result in holes in the WLAN coverage due to irregular coverage “footprints” of the APs at an additional cost of a reduction in maximum throughput of the system.
Thus, even if throughput can be increased, the network operator must continually adjust parameters of the WiFi network, such as power, frequency, and location. This increases the complexity in setting up and maintaining an optimal network.
Another challenge in deploying WiFi networks is the need for wiring. Each access point must be fed by a wire through regular network infrastructure. Even when LAN wiring already exists, it seldom fits the specific needs of radio based network, e.g., connecting socket locations are normally at lower sections of the walls while the location of access points is desired to be at the highest places (for better radio coverage). In many cases, people prefer to segregate the radio network from the wire-line network for security reasons. If the radio network could be supported by wireless backhaul instead, deployment could be less expensive and flexible. Significant amount of art has been published relative to this subject, such as mesh networks. However, mesh networks or any wireless backhaul that relies on native 802.11 standards suffers from prohibitive bandwidth loss. When one node in such a network is active, all other nodes around it must be silent, hence unable to communicate. A method to increase the transmission efficiency by overcoming this shortfall is required to implement efficient wireless backhaul. The deficiencies of WiFi in producing multi-hop (“mesh networks”) have been described in many papers. An example is “Revealing the Problems of 802.11 Medium Access Control in Multi-hop Wireless Ad Hoc Networks” by Shugong Xu and Tarek Saabdawi, published in “Computer Networks” magazine in 2002, that emphasized particularly the difficulties of TCP/IP in this environment.
In general, ad hoc, distributed control wireless networks, in particular 802.11 based, are not very suitable to multi-hop communications. An example of shared media (point-to-multipoint) based protocol that is used is the cable modem standard (DOCSIS). This protocol is a clear example of centralized control in shared media. Elements of this protocol were adopted by the WWAN industry (802.16).
Accordingly, there is a need in the art for improved techniques for scheduling transmissions in wireless networks, such as WiFi, that avoids the disadvantages of conventional methods discussed above.